Hypoxia induces leptin gene expression and secretion in human preadipocytes: differential effects of hypoxia on adipokine expression by preadipocytes

Regulation of HIF-1{alpha} activity in adipose tissue by obesity-associated factors: adipogenesis, insulin, and hypoxia

The transcription factor HIF-1α activity is increased in adipose tissue to contribute to chronic inflammation in obesity. However, its upstream and downstream events remain to be characterized in adipose tissue in obesity. We addressed this issue by investigating adipocyte HIF-1α activity in response to obesity-associated factors, such as adipogenesis, insulin, and hypoxia. In adipose tissue, both HIF-1α mRNA and protein were increased by obesity. The underlying mechanism was investigated in 3T3-L1 adipocytes. HIF-1α mRNA and protein were augmented by adipocyte differentiation. In differentiated adipocytes, insulin further enhanced HIF-1α in both levels. Hypoxia enhanced only HIF-1α protein, not mRNA. PI3K and mTOR activities are required for the HIF-1α expression. Function of HIF-1α protein was investigated in the regulation of VEGF gene transcription. ChIP assay shows that HIF-1α binds to the proximal hypoxia response element in the VEGF gene promoter, and its function is inhibited by a corepressor composed of HDAC3 and SMRT. These observations suggest that of the three obesity-associated factors, all of them are able to augment HIF-1α protein levels, but only two (adipogenesis and insulin) are able to enhance HIF-1α mRNA activity. Adipose tissue HIF-1α activity is influenced by multiple signals, including adipogenesis, insulin, and hypoxia in obesity. The transcriptional activity of HIF-1α is inhibited by HDAC3-SMRT corepressor in the VEGF gene promoter.

Immune cells in adipose tissue: key players in metabolic disorders

Obesity, defined as the excess development of adipose tissue, is an important risk factor for metabolic and cardiovascular diseases such as type 2 diabetes, hypertension and atherosclerosis. Over the past few years, metabolic inflammation has emerged as a major process underlying the link between obesity and its associated pathologies. Adipose tissue appears to play a primary and crucial role as a source and site of inflammation. Accumulation of immune cells within adipose tissue occurs in obese conditions. The present review focuses on the relationship between adipose tissue and immune cells, including macrophages, dendritic cells, T and B lymphocytes, and natural killer cells, in both the physiological state and under obese conditions. The factors involved in the accumulation of both myeloid and lymphoid cells in adipose tissue are also described. In addition, the role of adipose-tissue immune cells on adipocyte metabolism and cells of the adipose tissue stromal-vascular fraction are discussed, with particular emphasis on the cross-talk between macrophages and adipocytes, together with recent reports of T lymphocytes in adipose tissue.

New insights into adipose tissue dysfunction in insulin resistance

In a state of caloric excess, adipose tissue plays an essential role by storing lipids. Its expandability determines the onset of metabolic syndrome (central obesity, dyslipidemia, glucose intolerance and hypertension). When the adipocyte endoplasmic reticulum is no longer capable of processing the excess nutrients, the so-called “endoplasmic reticulum stress” develops. This triggers efflux of free fatty acids from adipocytes into the circulation and causes triglyceride overload in skeletal muscle, liver and pancreas. Adipose tissue hypoxia then develops, due to the failure of vasculature to expand with adipocyte hypertrophy. Increased catabolism in mitochondria leads there to oxidative stress. Both phenomena cause deranged adipokine secretion and low-grade inflammation. Inflammatory cytokines, reactive oxygen species and ectopic lipid deposition are the main mediators of insulin resistance and vascular impairment, which both lead finally to diabetes type 2 and cardiovascular disease. Recently, fibrosis of adipose tissue was also demonstrated in obesity, contributing to the interplay of deleterious factors forcing inflammation. The present paper reviews recent evidence for adipose tissue dysfunction, trying to define causes and consequences. In conclusion, insulin resistance and associated complications originate from excess lipids, which cannot be stored without limit in adipose tissue, thus affecting its integrity and adipokine secretion.

Macrophage-secreted factors inhibit ZAG expression and secretion by human adipocytes

Zinc-alpha2-glycoprotein (ZAG), a novel adipokine, is downregulated in adipose tissue in obesity, a state characterized by increased adipose tissue macrophage infiltration and chronic low-grade inflammation. This study investigated whether macrophage-secreted factors and TNF-alpha, a major product of macrophages, modulate ZAG expression and secretion by human adipocytes. ZAG was produced primarily by adipocytes, and not by preadipocytes and macrophages. Incubation of preadipocytes with macrophage-conditioned medium for up to 12 days decreased ZAG mRNA and protein release, and the expression of adipogenic markers (PPARgamma and C/EBPalpha). Adipocytes treated with macrophage-conditioned medium for 24h displayed significant reductions in ZAG mRNA and release. Chronic TNF-alpha treatment let to significant decreases in ZAG expression and secretion, but marked upregulation of pro-inflammatory cytokines and chemokines (IL-6, leptin, IL-8, MCP-1 and RANTES) in adipocytes. These findings suggest that macrophage-associated inflammation may play a significant role in the downregulation of ZAG in adipose tissue in obesity.

Chronic inflammation in obesity and the metabolic syndrome

The increasing incidence of obesity and the metabolic syndrome is disturbing. The activation of inflammatory pathways, used normally as host defence, reminds the seriousness of this condition. There is probably more than one cause for activation of inflammation. Apparently, metabolic overload evokes stress reactions, such as oxidative, inflammatory, organelle and cell hypertrophy, generating vicious cycles. Adipocyte hypertrophy, through physical reasons, facilitates cell rupture, what will evoke an inflammatory reaction. Inability of adipose tissue development to engulf incoming fat leads to deposition in other organs, mainly in the liver, with consequences on insulin resistance. The oxidative stress which accompanies feeding, particularly when there is excessive ingestion of fat and/or other macronutrients without concomitant ingestion of antioxidant-rich foods/beverages, may contribute to inflammation attributed to obesity. Moreover, data on the interaction of microbiota with food and obesity brought new hypothesis for the obesity/fat diet relationship with inflammation. Beyond these, other phenomena, for instance psychological and/or circadian rhythm disturbances, may likewise contribute to oxidative/inflammatory status. The difficulty in the management of obesity/metabolic syndrome is linked to their multifactorial nature where environmental, genetic and psychosocial factors interact through complex networks.

Two-photon microscopy for non-invasive, quantitative monitoring of stem cell differentiation

Background

The engineering of functional tissues is a complex multi-stage process, the success of which depends on the careful control of culture conditions and ultimately tissue maturation. To enable the efficient optimization of tissue development protocols, techniques suitable for monitoring the effects of added stimuli and induced tissue changes are needed.

Methodology/Principal Findings

Here, we present the quantitative use of two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) as a noninvasive means to monitor the differentiation of human mesenchymal stem cells (hMSCs) using entirely endogenous sources of contrast. We demonstrate that the individual fluorescence contribution from the intrinsic cellular fluorophores NAD(P)H, flavoproteins and lipofuscin can be extracted from TPEF images and monitored dynamically from the same cell population over time. Using the redox ratio, calculated from the contributions of NAD(P)H and flavoproteins, we identify distinct patterns in the evolution of the metabolic activity of hMSCs maintained in either propagation, osteogenic or adipogenic differentiation media. The differentiation of these cells is mirrored by changes in cell morphology apparent in high resolution TPEF images and by the detection of collagen production via SHG imaging. Finally, we find dramatic increases in lipofuscin levels in hMSCs maintained at 20% oxygen vs. those in 5% oxygen, establishing the use of this chromophore as a potential biomarker for oxidative stress.

Conclusions/Significance

In this study we demonstrate that it is possible to monitor the metabolic activity, morphology, ECM production and oxidative stress of hMSCs in a non-invasive manner. This is accomplished using generally available multiphoton microscopy equipment and simple data analysis techniques, such that the method can widely adopted by laboratories with a diversity of comparable equipment. This method therefore represents a powerful tool, which enables researchers to monitor engineered tissues and optimize culture conditions in a near real time manner.

SPARC: a key player in the pathologies associated with obesity and diabetes

SPARC (secreted protein acidic and rich in cysteine, also known as osteonectin or BM-40) is a widely expressed profibrotic protein with pleiotropic roles, which have been studied in a variety of conditions. Notably, SPARC is linked to human obesity; SPARC derived from adipose tissue is associated with insulin resistance and secretion of SPARC by adipose tissue is increased by insulin and the adipokine leptin. Furthermore, SPARC is associated with diabetes complications such as diabetic retinopathy and nephropathy, conditions that are ameliorated in the Sparc-knockout mouse model. As a regulator of the extracellular matrix, SPARC also contributes to adipose-tissue fibrosis. Evidence suggests that adipose tissue becomes increasingly fibrotic in obesity. Fibrosis of subcutaneous adipose tissue may restrict accumulation of triglycerides in this type of tissue. These triglycerides are, therefore, diverted and deposited as ectopic lipids in other tissues such as the liver or as intramyocellular lipids in skeletal muscle, which predisposes to insulin resistance. Hence, SPARC may represent a novel and important link between obesity and diabetes mellitus. This Review is focused on whether SPARC could be a key player in the pathology of obesity and its related metabolic complications.

Adult human bone marrow- and adipose tissue-derived stromal cells support the formation of prevascular-like structures from endothelial cells in vitro

Inadequate vascularization of in vitro-engineered tissue constructs after implantation is a major problem in most tissue-engineering applications. In this study we evaluated whether adipose tissue-derived stromal cells (ASCs), similar to bone marrow-derived stromal cells (BMSCs), can support the organization of endothelial cells into prevascular-like structures using an in vitro model. In addition, we investigated the mechanisms leading to the support of endothelial organization by these cells. We cultured human umbilical vein endothelial cells (HUVECs), ASCs, and BMSCs either alone or in combination in fibrin-embedded spheroids for 14 days. We found that BMSCs and ASCs formed cellular networks that expressed alpha smooth muscle actin and, in the case of ASCs, also CD34. Further, BMSCs and ASCs secreted hepatocyte growth factor and tissue inhibitor of metalloproteinase 1 and 2. In addition, ASC-conditioned medium induced HUVEC outgrowth, whereas BMSC-conditioned medium and hepatocyte growth factor-supplemented medium did not. Finally, both BMSCs and ASCs supported HUVEC organization into prevascular-like structures when cocultured. Our results suggest that both BMSCs and ASCs can support the formation of prevascular-like structures in vitro. Further, our findings indicate that cell-cell contacts and reciprocal signaling play an important role in the formation of these prevascular structures.

Functional analyses reveal the greater potency of preadipocytes compared with adipocytes as endothelial cell activator under normoxia, hypoxia, and TNFalpha exposure

Obesity is associated with a state of chronic low-grade inflammation. Immune cells accumulate in white adipose tissue (WAT). The vascular endothelium plays an interactive role in these infiltration and inflammatory processes. Mature and hypertrophic adipocytes are considered as the major adipogenic cell type secreting proinflammatory cytokines in WAT. In contrast, the proinflammatory capacity of preadipocytes and their role in endothelial cell activation have been neglected so far. To gain new insights into this molecular and cellular cross-talk, we examined the proinflammatory expression and secretion of normoxia, hypoxia, and TNFalpha-treated human preadipocytes and adipocytes (SGBS cells) and their impact on human microvascular endothelial cell (HMEC-1) function. In this study, stimulation of HMEC-1 with conditioned media (CM) from preadipocytes increased endothelial ICAM-1 expression and monocyte adhesion but not adipocyte-CM. After hypoxia and TNFalpha stimulation of SGBS cells, adipocyte-CM induced and preadipocyte-CM enhanced the monocyte adhesion. Concordantly, the expression of proinflammatory adipokines was considerably higher in preadipocytes than in adipocytes. SGBS-CM upregulated the phosphorylation of three MAPK pathways, STAT1/3, and c-Jun in HMEC-1, whereas the NF-kappaB pathway was not affected. Inhibitor experiments showed that monocyte/endothelial cell-cell adhesion and endothelial ICAM-1 expression was JNK and JAK-1/STAT1/3 pathway dependent and revealed IL-6 as a major mediator in CM increasing monocyte/endothelial cell-cell adhesion via the STAT1/3 pathway. Our study shows that preadipocytes rather than adipocytes operate as potent activators of endothelial cells. This can be enhanced in preadipocytes and induced in adipocytes by TNFalpha and hypoxia in a manner similar to what may occur in WAT in the etiology of obesity.

Cellular hypoxia and adipose tissue dysfunction in obesity

Expansion of adipose tissue mass, the distinctive feature of obesity, is associated with low-grade inflammation. White adipose tissue secretes a diverse range of adipokines, a number of which are inflammatory mediators (such as TNFalpha, IL-1beta, IL-6, monocyte chemoattractant protein 1). The production of inflammatory adipokines is increased with obesity and these adipokines have been implicated in the development of insulin resistance and the metabolic syndrome. However, the basis for the link between increased adiposity and inflammation is unclear. It has been proposed previously that hypoxia may occur in areas within adipose tissue in obesity as a result of adipocyte hypertrophy compromising effective O2 supply from the vasculature, thereby instigating an inflammatory response through recruitment of the transcription factor, hypoxic inducible factor-1. Studies in animal models (mutant mice, diet-induced obesity) and cell-culture systems (mouse and human adipocytes) have provided strong support for a role for hypoxia in modulating the production of several inflammation-related adipokines, including increased IL-6, leptin and macrophage migratory inhibition factor production together with reduced adiponectin synthesis. Increased glucose transport into adipocytes is also observed with low O2 tension, largely as a result of the up-regulation of GLUT-1 expression, indicating changes in cellular glucose metabolism. Hypoxia also induces inflammatory responses in macrophages and inhibits the differentiation of preadipocytes (while inducing the expression of leptin). Collectively, there is strong evidence to suggest that cellular hypoxia may be a key factor in adipocyte physiology and the underlying cause of adipose tissue dysfunction contributing to the adverse metabolic milieu associated with obesity.

Leptin and the skin: a new frontier

Here, we examine the currently available information which supports that the adipokine, leptin, is a major player in the biology and pathology of mammalian skin and its appendages. Specifically, the potent metabolic effects of leptin and its mimetics may be utilized to improve, preserve and restore skin regeneration and hair cycle progression, and may halt or even partially reverse some aspects of skin ageing. Since leptin can enhance mitochondrial activity and biogenesis, this may contribute to the wound healing-promoting and hair growth-modulatory effects of leptin. Leptin dependent intracellular signalling by the Janus kinase 2 dependent signal transducer and activator of transcription 3, adenosine monophosphate kinase, and peroxisome proliferator-activated receptor (PPAR) gamma coactivator/PPAR converges to mediate mitochondrial metabolic activation and enhanced cell proliferation which may orchestrate the potent developmental, trophic and protective effects of leptin. Since leptin and leptin mimetics have already been clinically tested, investigative dermatology is well-advised to place greater emphasis on the systematic exploration of the cutaneous dimensions and dermatological potential of this pleiotropic hormone.

IL-33, a recently identified interleukin-1 gene family member, is expressed in human adipocytes

Inflammation occurs in adipose tissue in obesity. We have examined whether IL-33, a recently identified IL-1 gene family member, and its associated receptors are expressed in human adipocytes. IL-33, IL-1RL1 and IL-1RAP gene expression was observed in human visceral white fat, in preadipocytes and in adipocytes (SGBS cells). Treatment with TNFalpha for 24h induced a 6-fold increase in IL-33 mRNA level in preadipocytes and adipocytes. Time-course studies with adipocytes showed that the increase in IL-33 mRNA with TNFalpha was maximal (>55-fold) at 12h. This response was markedly different to IL-1beta (peak mRNA increase at 2h; 5.4-fold) and 1L-18 (peak mRNA increase at 6h; >1500-fold). Exposure of adipocytes to hypoxia (1% O(2), 24h) did not alter IL-33 mRNA level; in preadipocytes, however, there was a 3-fold increase. Human adipocytes and preadipocytes express IL-33, but the various IL-1 family members exhibit major differences in responsiveness to TNFalpha.

Hypoxia decreases insulin signaling pathways in adipocytes

OBJECTIVE

Obesity is characterized by an overgrowth of adipose tissue that leads to the formation of hypoxic areas within this tissue. We investigated whether this phenomenon could be responsible for insulin resistance by studying the effect of hypoxia on the insulin signaling pathway in adipocytes.

RESEARCH DESIGN AND METHODS

The hypoxic signaling pathway was modulated in adipocytes from human and murine origins through incubation under hypoxic conditions (1% O(2)) or modulation of hypoxia-inducible factor (HIF) expression. Insulin signaling was monitored through the phosphorylation state of several key partners of the pathway and glucose transport.

RESULTS

In both human and murine adipocytes, hypoxia inhibits insulin signaling as revealed by a decrease in the phosphorylation of insulin receptor. In 3T3-L1 adipocytes, this inhibition of insulin receptor phosphorylation is followed by a decrease in the phosphorylation state of protein kinase B and AS160, as well as an inhibition of glucose transport in response to insulin. These processes were reversible under normoxic conditions. The mechanism of inhibition seems independent of protein tyrosine phosphatase activities. Overexpression of HIF-1alpha or -2alpha or activation of HIF transcription factor with CoCl(2) mimicked the effect of hypoxia on insulin signaling, whereas downregulation of HIF-1alpha and -2alpha by small interfering RNA inhibited it.

CONCLUSIONS

We have demonstrated that hypoxia creates a state of insulin resistance in adipocytes that is dependent upon HIF transcription factor expression. Hypoxia could be envisioned as a new mechanism that participates in insulin resistance in adipose tissue of obese patients.

Emerging role of adipose tissue hypoxia in obesity and insulin resistance

Recent studies consistently support a hypoxia response in the adipose tissue in obese animals. The observations have led to the formation of an exciting concept, adipose tissue hypoxia (ATH), in the understanding of major disorders associated with obesity. ATH may provide cellular mechanisms for chronic inflammation, macrophage infiltration, adiponectin reduction, leptin elevation, adipocyte death, endoplasmic reticulum stress and mitochondrial dysfunction in white adipose tissue in obesity. The concept suggests that inhibition of adipogenesis and triglyceride synthesis by hypoxia may be a new mechanism for elevated free fatty acids in the circulation in obesity. ATH may represent a unified cellular mechanism for a variety of metabolic disorders and insulin resistance in patients with metabolic syndrome. It suggests a new mechanism of pathogenesis of insulin resistance and inflammation in obstructive sleep apnea. In addition, it may help us to understand the beneficial effects of caloric restriction, physical exercise and angiotensin II inhibitors in the improvement of insulin sensitivity. In this review article, literatures are reviewed to summarize the evidence and possible cellular mechanisms of ATH. The directions and road blocks in the future studies are analyzed.

Macrophage infiltration into adipose tissue: initiation, propagation and remodeling

It has long been known that adipose tissue in obesity is in a heightened state of inflammation. Recently, our understanding of this has been transformed by the knowledge that immune cells such as macrophages and T cells can infiltrate adipose tissue and are responsible for the majority of inflammatory cytokine production. These seminal findings have opened up a new area in biology that is garnering the interest of scientists involved in research relating to cell motility, inflammation, obesity, physiology, diabetes and cardiovascular disease. Some important general questions relevant to this field are: how are macrophages recruited to adipose tissue in obesity? What are the physiological consequences of macrophage-adipocyte interactions? Do these inflammatory macrophages contribute to pathophysiological conditions associated with obesity, such as insulin resistance, dyslipidemia, diabetes and cardiovascular disease? This review focuses on the first of these important questions.